Gametogenesis and Pre-ebryonic life by Shapi. MDpdf

ANATOMY
• EMBRYOLOGY
• GAMETOGENESIS AND PRE-
EMBRYONIC LIFE
• Dr. Chongo Shapi, (BSc. HB, MBChB)
• Medical Doctor.
08/11/2022 Dr. Chongo Shapi, BSc. HB, MBChB 1

INTRODUCTION
GAMETOGENESIS
• Spermatogenesis.
• Oogenesis.
PRE-EMBRYONIC LIFE
• Fertilization.
• Implantation &
• Blastocyst formation

LEARNING OBJECTIVES
• Compare/contrast spermatogenesis & oogenesis.
• Explain how a diploid zygote is produced (fertilization).
• Explain process and duration of implantation.
• Explain process of blastocyst formation and its
significance.

GAMETOGENESIS

SPERMATOGENESIS
• Occurs in the seminiferous tubules of the
testes
• Literally, the formation of sperm takes 65 to
70 days.

• Spermatogonia produce 2
kinds of daughter cells
– Type A remain outside blood-
testis barrier and produce more
daughter cells until death
– Type B differentiate into
primary spermatocytes
• Cells must pass through
BTB to move inward
toward lumen - new tight
junctions form behind
these cells
• meiosis I → 2 secondary
spermatocytes
• meiosis II → 4
spermatids
SPERMATOGENESIS

SPERMATOGENESIS
➢ Primary spermatocytes
• These are the basic cells, where
process begins
– Diploid (2n)
– Produced continuously by
mitosis
➢ Secondary spermatocytes
These are haploid (n)
➢ Sperm cells
Also haploid, are produced
when secondary
spermatocytes divide

OOGENESIS
• Occurs in the ovaries.
• Formation of oocytes/eggs.
• Begins prior to birth!
– Spans all of pre-reproductive & reproductive life
• Begins with one cell
– Which becomes two
• Then finally, four

OOGENESIS
➢ Primary oocyte
• These are present at birth
• Stay dormant until sometime after
puberty
• Diploid (2n)
– 46 chromosomes
➢ First polar
• Results from meiosis I
• Smaller (has less cytoplasm)
Secondary oocytes
• Released by ovulation
– Happens roughly every 28 days.
➢ Second polar body
• Results from meiosis II
• Smaller (has less cytoplasm)
• Contains the “loser” nucleus.

OVARIAN CYCLE
• Monthly series of events associated with
the maturation of an oocyte/egg.
• Follicular phase – period of follicle growth
(days 1–14).
• Luteal phase – period of corpus luteum
activity (days 14–28).
• Ovulation occurs mid-cycle.

SEXUAL CYCLE
• Averages 28 days, ranges from 20 to 45
• Hormone cycle: hierarchy of control
– hypothalamus→ pituitary → ovaries → uterus
• Follicular phase (2 weeks)
– menstruation occurs during first 3 to 5 days of cycle
– uterus replaces lost endometrium and follicles grow
• Luteal phase (2 weeks)
– corpus luteum stimulates endometrial thickening
– endometrium lost without pregnancy

HORMONE INTERACTION DURING THE
OVARIAN CYCLE
• Day 1 – GnRH stimulates the release of FSH and LH
• FSH and LH stimulate follicle growth and maturation,
and low-level estrogen release
• Rising estrogen levels:
– Inhibit the release of FSH and LH
– Prod the pituitary to synthesize and accumulate these
gonadotropins
• Estrogen levels increase and high estrogen levels
have a positive feedback effect on the pituitary,
causing a sudden surge of LH.

CONT’
• The LH spike stimulates the primary oocyte to
complete meiosis I, and the secondary oocyte
continues on to metaphase II
• Day 14 – LH triggers ovulation
• LH transforms the ruptured follicle into a
corpus luteum, which produces inhibin,
progesterone, and estrogen.

CONT’
• These hormones shut off FSH and LH release
and declining LH ends luteal activity
• Days 26-28 – decline of the ovarian hormones
– Ends the blockade of FSH and LH
– The cycle starts anew

FOLLICULAR PHASE
• The primordial follicle, directed by the oocyte,
becomes a primary follicle
• Primary follicle becomes a secondary follicle
– The theca folliculi and granulosa cells cooperate to
produce estrogens
– The zona pellucida forms around the oocyte
– The antrum is formed.

CONT’
• The secondary follicle becomes a vesicular
follicle
– The antrum expands and isolates the oocyte and
the corona radiata
– The full size follicle (vesicular follicle) bulges from
the external surface of the ovary
– The primary oocyte completes meiosis I, and the
stage is set for ovulation

Ovarian Cycle - Follicular Phase
• Menstruation (day 1) to ovulation(14) (variable)
• Difficult to predict date of ovulation
• Contains menstrual and preovulatory phases

Ovarian Cycle - Preantral Phase
• Discharge of menstrual fluid (days 1-5)
• Before follicle develops antrum
– primordial and primary follicles

Ovarian Cycle - Antral Phase
• Day 6 to 14, one dominant follicle advances to mature (graafian)
follicle; secretes estrogen

OVULATION
• Ovulation occurs when the ovary wall ruptures
and expels the secondary oocyte
• Mittelschmerz – a twinge of pain sometimes
felt at ovulation
• 1-2% of ovulations release more than one
secondary oocyte, which if fertilized, results in
fraternal twins

Ovarian Cycle - Ovulation
• Mature follicle ruptures, releases oocyte influenced by LH

OVULATION CONT’
• Middle pain
• Increase basal temperature
• Stimulation of gonadotropin and multiple pregnancy.
CORPUS LUTEUM
• After ovulation, granulosa cells remaining in the wall of the
ruptured follicle, together with cells from the theca interna, are
vascularized by surrounding vessels. Under the influence of LH,
these cells develop a yellowish pigment and change into lutean
cells, which form the corpus luteum and secrete the hormone
progesterone .
• Progesterone, together with estrogenic hormones, causes the
uterine mucosa to enter the progestational or secretory stage in
preparation for implantation of the embryo.

OOCYTE TRANSPORT
Figure 2.4 Relation of fimbriae and ovary. Fimbriae collect the
oocyte and sweep it into the uterine tube.

CORPUS LUTEUM
CORPUS ALBICANS
• If fertilization does not occur, the corpus luteum
reaches maximum development approximately 9
days after ovulation.
• Subsequently, the corpus luteum shrinks because
of degeneration of lutean cells and forms a mass
of fibrotic scar tissue, the corpus albicans.
• Simultaneously, progesterone production
decreases, precipitating menstrual bleeding

CORPUS LUTEUM CONT’
• If the oocyte is fertilized, degeneration of the corpus
luteum is prevented by human chorionic gonadotropin
(hCG), a hormone secreted by the syncytiotrophoblast of
the developing embryo.
• The corpus luteum continues to grow and forms the corpus
luteum of pregnancy (corpus luteum graviditatis).
• Yellowish luteal cells continue to secrete progesterone until
the end of the fourth month; thereafter, they regress slowly
as secretion of progesterone by the trophoblastic
component of the placenta becomes adequate for
maintenance of pregnancy.
NB: Removal of the corpus luteum of pregnancy before the
fourth month usually leads to abortion.

FERTILIZATION

BEFORE FERTILIZATION
• Capacitation is a period of conditioning in the female reproductive
tract that in the human lasts approximately 7 hours. Much of this
conditioning, which occurs in the uterine tube, entails epithelial
interactions between the sperm and mucosal surface of the tube.
• During this time, a glycoprotein coat and seminal plasma proteins
are removed from the plasma membrane that overlies the
acrosomal region of the spermatozoa.
NB: Only capacitated sperm can pass through the corona cells and
undergo the acrosome reaction.
• Acrosome reaction, which occurs after binding to the zona
pellucida, is induced by zona proteins. This reaction culminates in
the release of enzymes needed to penetrate the zona pellucida,
including acrosin and trypsin-like substances.

FERTILIZATION
• Sperm penetrates
secondary oocyte
– Triggers meiosis II
– Oocyte’s nucleus divides
• One nucleus stays,
combines with sperm’s
nucleus
• Other nucleus leaves

PHASES OF FERTILIZATION
• Phase 1, penetration of the corona radiata (Of the 200 to
300 million spermatozoa deposited, only 300 to 500 reach
the site of fertilization. Only one of these fertilizes the egg.)
• Phase 2, penetration of the zona pellucida. The zona is a
glycoprotein shell surrounding the egg that facilitates and
maintains sperm binding and induces the acrosome
reaction.
➢ Both binding and the acrosome reaction are mediated by
the ligand ZP3, a zona protein.
➢ Release of acrosomal enzymes (acrosin) allows sperm to
penetrate the zona, thereby coming in contact with the
plasma membrane of the oocyte,

CONT’
➢Contact results in release of lysosomal enzymes
from cortical granules lining the plasma
membrane of the oocyte, these enzymes alter
properties of the zona pellucida (zona reaction)
to prevent sperm penetration;
• Phase 3, fusion of the oocyte and sperm cell
membranes. In the human, both the head and tail
of the spermatozoon enter the cytoplasm of the
oocyte, but the plasma membrane is left behind
on the oocyte surface.

THE EGG RESPONSE
1. Cortical and zona reactions. As a result of CR & ZR,
(a) Oocyte membrane becomes impenetrable to other
spermatozoa, and
(b) Zona pellucida alters its structure and composition to
prevent sperm binding and penetration.
These reactions prevent polyspermy (penetration of more
than one spermatozoon into the oocyte).
2. Resumption of the second meiotic division. The oocyte
finishes its second meiotic division immediately after entry of
the spermatozoon.
3. Metabolic activation of the egg.

FERTILIZATION!!! WHAT IS NEXT???
• Restoration of the diploid number of
chromosomes,
• Determination of the sex of the new individual.
An X-carrying sperm produces a female (XX)
embryo, and a Y-carrying sperm produces a male
(XY) embryo. Hence, the chromosomal sex of the
embryo is determined at fertilization.
• Initiation of cleavage. Without fertilization, the
oocyte usually degenerates 24 hours after
ovulation.

CLEAVAGE

BLASTOCYST FORMATION & IMPLANTATION

Uterus at Time of Implantation

Uterus at Time of Implantation cont’

Molecular bases & absence of fertilization

CLINICAL CORRELATES
1. Barrier techniques of contraception (male condom-made
of latex and often containing chemical spermicides, and
the female condom-made of polyurethane. Other barriers
include the diaphragm, the cervical cap, and the
contraceptive sponge.
2. Contraceptive pill, a combination of estrogen and the
progesterone analogue-progestin e.g. Depo-Provera (5yrs
SD or 23months IM), which together inhibit ovulation but
permit menstruation.
• male “pill” i.e syntheetic androgen prevents both LH and
FSH secretion and either stops sperm production (70–90%
of men) or reduces it to a level of infertility.
• drug RU-486 (mifepristone) causes abortion if it is
administered within 8 weeks of the previous menses.
• Vasectomy and tubal ligation are effective means of
contraception, and both procedures are reversible, although
not in every case.

INFERTILITY
• Infertility is a problem for 15% to 30% of couples. Male
infertility may be a result of insufficient numbers of
sperm or poor motility.
• Normally, the ejaculate has a volume of 3 to 4 ml, with
approximately 100 million sperm per ml. Males with 20
million sperm per ml or 50 million sperm per total
ejaculate are usually fertile.
• very few live sperm (oligozoospermia) or even no live
sperm (azoospermia), can be overcome using
1. intracytoplasmic sperm injection (ICSI).
2. gamete intrafallopian transfer (GIFT)
3. zygote intrafallopian transfer (GIFT)

INFERTILITY CONT’
• Infertility in a woman may be due to a number of
causes, including;
1. Occluded oviducts (most commonly caused by pelvic
inflammatory disease).
2. Hostile cervical mucus.
3. Immunity to spermatozoa.
4. Absence of ovulation, and others.
Treatment……
1. In vitro fertilization (IVF)
2. gamete intrafallopian transfer (GIFT)
3. zygote intrafallopian transfer (ZIFT)

THANK YOU

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